The ongoing demand for improved efficiency has resulted in the design of modern gas turbine engines operating at increasingly high temperatures. Generally, when the combustion gas temperature exceeds a value at which a structural material begins to degrade, the designer is forced to select a different material having a higher safe operating temperature, to provide a cooling mechanism for the material, and/or to coat the structural material with a non-structural thermal barrier coating. Special superalloy and ceramic materials have been developed for use at the high temperatures generated by hot combustion gasses in a gas turbine engine. For example, A-N720 is an oxide-oxide ceramic matrix composite (CMC) material available from COI Ceramics, Inc. that can safely function without significant degradation at temperatures up to about 1,200° C. For temperatures exceeding this value, active or passive cooling techniques may be used to protect the material. Alternatively or in combination with cooling, a thermal barrier coating may be applied to protect the material from the environment. U.S. Pat. No. 6,013,592, commonly assigned with the present invention, describes one such ceramic thermal barrier coating material applied to a ceramic matrix composite substrate.
The use of a thermal barrier coating creates a new set of concerns for the designer. First, the coating process adds cost and the coating adds weight to the component. Furthermore, failure of the coating can lead to failure of the component, thus potentially detracting from the statistical reliability of the system. A thermal barrier coating must remain firmly bonded to the substrate in order to be effective. One mode of coating failure is spalling due to differential thermal expansion between the coating and the substrate. U.S. Pat. No. 6,013,592 addresses this problem by varying the composition of ceramic spheres within the coating to adjust the coefficient of thermal expansion to a desired value. Nonetheless, thermal barrier coatings with improved reliability and reduced cost are desired.